A bus system for a local operation network for automotive applications is disclosed in DE-OS 35 06 118. The asynchronous bus access method used there is carrier sense multiple access with collision detection (CSMA/CD), wherein contention between two or more subscribers is resolved by an arbitration mechanism at bit level.
In the CSMA/CD method, each bus station having data to be transmitted places its data on the bus if the bus is not currently being used by any other station (carrier sense). If two bus stations, taking account of signal propagation delays, transmit quasi-simultaneously, collisions will occur which must be detected by the stations by listening to the bus. These stations then take appropriate steps to eliminate the collision. This has the disadvantage that all transmitted data are lost, so that the bus stations must transmit their data anew. Another disadvantage of such an asynchronous CSMA/CD method is that the transmission of a given bit rate for a given bus station cannot be guaranteed.
Also known is an asynchronous token access method, in which access to the data bus is controlled by a token representing an access authorization. Only that bus station may place data on the bus which is in possession of the token. The single token of the system is passed from bus station to bus station, with appropriate steps having to be initiated if the token is lost or is multiplied by a faulty operation in the bus system. This asynchronous token bus method has the disadvantage that it can guarantee only limitedly the transmission of a given bit rate for a given station.
Frame-structured bus systems are known which have synchronous bus access mechanisms, whose organization is essentially channel-oriented: One bus station generates data frames of equal bit length which each have a plurality of time slots. Each of the other bus stations is assigned one time slot during which it can insert data into the data frame. The constancy of the frame length and the predetermined division of the time slots within a data frame ensure that each of the stations can transmit a defined, predetermined bit rate. A disadvantage of this rigid frame and time-slot structure is that such a synchronous access mechanism is of only low efficiency if the amounts of data to be transmitted by the bus stations vary widely. This situation occurs, for example, in a local operation network for automotive applications, where audio and/or video data are fed from a bus station into the operation network: During the operating time, high data rates are constantly present on the data bus of the operation network, while no data have to be transmitted when the audio/video set is off. The fixed structure of the time slots within a predetermined data structure then results in the time slots assigned to the audio/video bus station remaining unused outside the operating time of the radio. Due to this insufficient utilization of transmission capacity, the efficiency of the frame-structured bus system is drastically reduced.
The known prior art bus systems have the disadvantage of being not flexible enough to meet the increased requirements in the automotive field. In a modern local operation network for automotive applications, not only the standard data accruing in each automobile for ensuring the functionality of the auto and for monitoring and controlling functional groups are to be transmitted. To meet customers' wishes, every major automobile manufacturer offers a number of extras. Therefore, apart from the standard data, a number of further data have to be transmitted through the local operation network which are necessary to control the extras or accrue during operation of this additional equipment. When equipping an automobile with such extras, the problem arises that in different models, these extras have to be mounted in different places. As a result, propagation delays between these stations and the central star coupler vary from model to model. The prior art bus system for an automotive local operation network does not allow these different, model-dependent propagation delays to be taken into account. Therefore, its field of use is greatly limited, so that it is not universally applicable.
Also, during integration of the different extras in the single operation network, the problem arises that for the joint transmission of signals typically occurring in automobiles and of signals of the extra equipment (e.g., electronic on-board office), synchronous and asynchronous data have to be switched jointly, with the data rates varying from a few bauds to several Mbauds. To reach synchronism in the network, it is then also necessary to compensate for differences in propagation delay.
Furthermore, in a fiber-optic local operation network, the problem arises that transmission times change as a result of the natural aging of the glass fibers used. The prior art bus system for the prior art local operation network cannot take this into account, either.